18 MANUAL OF TREE DISEASES 



diagnosis of the trouble, since usually no determinative symp- 

 toms are present. The ultimate factor of importance in most 

 cases is an insufficient supply of water available to com- 

 pensate for the loss by transpiration. Conditions bringing 

 about such injuries may occur at any time of year when the 

 leaves are on the tree. 



Atmospheric impurities are mostly injurious in and near 

 cities where gases of various kinds are allowed to escape with- 

 out restriction. Soot from ordinary coal-smoke may cause 

 damage if quantities of it are continually falling on trees. 



The more abnormal the surroundings in which trees are 

 placed, the more important and common become the injuries 

 due to faulty nutrition. The symptoms of malnutrition, in- 

 dependent of other causes, are generally poor growth and the 

 chlorosis or yellowing of the leaves. In addition to these types 

 of injury which show prominently in the leaves, any diseased 

 condition of the roots, trunk or branches which interferes 

 with the necessary conducting of food materials to the leaves 

 results in injury to the leaves. Consequently in diagnosing 

 leaf-injury, the leaves alone should not be examined but the 

 condition of the branches, trunk and roots should be studied 

 to see whether they show the primary symptoms. The 

 diseases of leaves caused by specific pathogenes will be found 

 discussed under the kind of tree attacked. 



Winter-Drying 



Caused by simultaneous low soil temperatiires and high air tem- 

 peratures 



Conifers which retain their foliage throughout the winter 

 frequently suffer injury to the leaves of a part or all of the 

 tree due to drying. After severe winters, trees may be found 

 in exposed places which have been killed outright. The injury 

 is very conspicuous and where generally prevalent it causes 



